Optical interconnects have been proposed as a replacement for electrical interconnects because the loss in electrical wires at high frequencies is becoming a major limitation in information processing devices. To be practical, optical interconnects must therefore consume very little power. For example, one study found that in order for an on-chip optical interconnect to be useful, the entire optical link must consume less than 10 fJ/bit. Semiconductor nanocavity lasers are promising optical sources for this application because they consume little power due to the small active volume and the Purcell effect threshold reduction. The optically pumped photonic crystal (PC) nanocavity laser has been extensively studied because it has been shown to have nanowatt thresholds and high speed modulation rates. In addition, it has been demonstrated that it can operate in continuous wave mode at room temperature, and can be easily integrated with passive elements, such as PC waveguides, making them promising for optoelectronic integrated circuits. However, in order for them to be practical, electrical pumping techniques must be developed.
The main challenge of electrically pumping PC membrane nanocavities is how to efficiently inject current into the cavity region. A vertical p-i-n junction in the membrane has been used to electrically pump GaAs PC cavities and to demonstrate reduction of emission rate for cavity coupled electroluminescence (EL). However, due to the limited current spreading ability of the thin conductive layers, most of the electroluminescence is not coupled to the cavity. A PC nanocavity can be efficiently pumped by using a central current post, and lasing has been demonstrated using this technique (Park et al., Science v305 n5689 pp 1444-1447, 2004). One disadvantage of the approach of Park et al. is that the fabrication is complicated, requiring a precisely timed undercut step, and an arbitrary PC design cannot be used since the size and position of the PC holes determine the current post size. In addition, the lasers had a high threshold compared to other electrically pumped microcavity lasers such as micropillars, suggesting that the PC lasers suffered from undesirable leakage current. Thus, practical electrical pumping of PC semiconductor devices has not yet been demonstrated.
One example of PC nanocavities in the art is the work of Tanabe et al., (e.g., as described in Applied Physics Letters v96, pp 101103-1 to 101103-3 and in Optics Express v17n25, pp 22505-22513) relating to PC photodetectors and modulators. Other examples of PC nanocavities in the art include U.S. Pat. Nos. 5,784,400, 7,079,240 and 7,492,979, where lasing in such nanocavities is considered. However, as indicated above, electrical pumping of PC nanocavities is difficult, and none of these patent references demonstrate electrical pumping of a PC laser.